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Crop yields cut by almost half due to India’s dirty air: study

04 November, 2014

The Guardian

Study finds that 90 per cent of falls in production of wheat and rice over 30 years could be attributed to black carbon and ground level ozone

Air pollution in India has become so severe that crop yields are being cut by almost half, scientists have found.

Researchers analysed yields for wheat and rice alongside pollution data, and concluded significant decreases in yield could be attributed to two air pollutants, black carbon and ground level ozone. The finding could also be relevant to farmers in China, as well as having implications for global food security as India is a major rice exporter.

Black carbon is mostly caused by rural cooking stoves, and ozone forms as a result of motor vehicle exhaust, industrial emissions, and chemical solvents reacting in the atmosphere in the presence of sunlight. Both are “short-lived climate pollutants” that exist locally for weeks to months, with ozone damaging the leaves on plants and black carbon reducing the amount of sunlight they receive.

The study looked at both the effects of climate change and the two pollutants on crop yields.

“While temperature has gone up in the last three decades, the levels of smog and pollution have changed much more dramatically,” said Jennifer Burney, an environmental scientist at the University of California, San Diego, and co-author of the paper, published in Proceedings of the National Academy of Sciences. “But this was the first time anyone looked at historical data to show these pollutants are having tremendous impacts on crops.”

Comparing crop yields in 2010 to what they would be expected to be if temperature, rainfall and pollution remained at 1980 levels, the researchers showed that yields for wheat were on average 36 per cent lower than they otherwise would have been, while rice production decreased by up to 20 per cent. In some higher population states, wheat yields were as much as 50 per cent lower.

Using modelling to account for the effects of temperature increase and precipitation changes in that time, they were able to show that 90 per cent of this loss is attributable to the impact of the two pollutants.

The results are specific to India’s seasonal patterns, the crops, and its pollution levels, but may extend to other places with similar problems. Chinese scientists warned in February that air pollution is slowing photosynthesis in plants, with effects “somewhat similar to a nuclear winter”.

Previous studies had used experimental data looking at the impact of ozone on plants to extrapolate potential losses, but this is the first study to use actual historical agricultural and emissions data to account for lower crop yields.

“Overall I think it’s a great paper,” said Stanford University agricultural ecologist David Lobell. “I think in both India and China there is growing recognition of the toll poor air quality has on agriculture. This study will certainly add to that.”

Lobell and Burney both point out that because black carbon and ozone are short-lived pollutants, they present a clear opportunity for tackling climate change. While long-lived greenhouse gases like carbon dioxide and nitrous oxides can persist in the atmosphere for decades, addressing sources of the short-lived pollutants will have more immediately perceptible effects.

Measures such as improved cooking stove technology for rural areas, or cleaner coal consumption and diesel filters on trucks in urban ones, could go a long way to improving the impact on agricultural yields.

“Our thought is that these are more politically tractable points of entry for making a meaningful change in the climate,” said Burney. “There’s a really local benefit in taking on some sort of costly action.”

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